The present invention relates to a semiconductor device and a fabrication method therefor and, more particularly, to a MOS transistor having a thin gate insulating film and a low-resistance gate electrode formed on the gate insulating film and a fabrication method therefor.
To implement a semiconductor device composed of a MOS transistor which is smaller in size, higher in integration, and operable with a lower voltage, it is necessary to reduce the resistances of the materials of a wire, an electrode, and the like each composing the semiconductor device and thereby reduce a delay time resulting from wiring resistance.
Accordingly, a multilayer film composed of a polysilicon film and a metal silicide film has been used for the gate electrode of the MOS transistor.
In a MOS transistor with an extremely small design rule of 0.10 xcexcm or less, however, a sufficiently reduced resistance can not be obtained with the gate electrode formed of the multilayer film composed of the polysilicon film and the metal silicide film. As a substitute, therefore, a metal gate process has been considered in which the gate electrode is formed of a refractory metal film such as a tungsten film.
Referring to FIGS. 15(a), (b), (c), and (d), a method of fabricating a semiconductor device according to a first conventional embodiment will be described, in which the gate electrode is formed by the metal gate process.
First, as shown in FIG. 15(a), an insulating film 11 for isolation and a p-type semiconductor region 12 are formed successively in a surface portion of a semiconductor substrate 10. A silicon oxide film 13, serving as a gate insulating film, is then formed on a region of the semiconductor substrate 10 surrounded by the insulating film 11 for isolation. Thereafter, a target made of tungsten is sputtered in an argon ambient, whereby a tungsten film 14 serving as a gate electrode is deposited over the entire surface of the semiconductor substrate 10.
Next, as shown in FIG. 15(b), a resist pattern 15 is formed on a region of the tungsten film 14 in which a gate electrode is to be formed. Then, as shown in FIG. 15(c), the tungsten film 14 and the silicon oxide film 13 masked with the resist pattern 15 is etched to form a gate electrode 14A and a gate oxide film 13A.
Next, as shown in FIG. 15(d), an n-type lightly doped region 16 is formed by implanting an n-type dopant by using the gate electrode 14A as a mask, followed by sidewalls 17 formed on the side surfaces of the gate electrode 14A. Thereafter, an n-type heavily doped region 18 is formed by implanting an n-type dopant by using the gate electrode 14A and sidewalls 17 as a mask. A heat treatment is then performed to activate the n-type lightly doped region 16 and the n-type heavily doped region 18.
Next, contacts, metal wires, and the like are formed, though they are not shown in the drawing. As a result, the semiconductor device having the gate electrode 13A made of tungsten is obtained.
It has been reported that, if the heat treatment is performed in a nitrogen ambient at a temperature of 900 to 1100xc2x0 C. for about 30 minutes after the formation of the gate electrode 14A composed of the tungsten film 13, the internal stress of the gate electrode 14A can be reduced and the reliability of the MOS transistor is improved thereby (N. Yamamoto, S. Iwata, and H. Kume, xe2x80x9cThe influence of Internal Stresses in Tungsten-Gate Electrodes on the Degradation of MOSFET Characteristics Caused by Hot Carriers: IEEE Trans, Electron Device, vol. ED-34, pp. 607-614 1987).
Hereinafter, a second conventional embodiment of the method of fabricating a semiconductor device disclosed in Japanese Unexamined Patent Publication No. 10-233505 will be described, in which the gate electrode is formed by the metal gate process.
In the second conventional embodiment, a nitrogen-containing tungsten film composed of a composite of tungsten and tungsten nitride is formed on a gate insulating film by sputtering a target made of tungsten in a nitrogen ambient. A heat treatment is then performed with respect to the nitrogen-containing tungsten film to diffuse nitrogen contained in the nitrogen-containing tungsten film, thereby preventing the reliability of the gate insulating film from deteriorating.
Since further scaling down of a semiconductor integrated circuit requires a thinner gate insulating film, the achievement of higher reliability is becoming increasingly important in a MOS transistor having an extremely thin gate insulating film. It is also known that the material of the gate electrode greatly affects the reliability of the gate insulating film.
In the case where the tungsten film is used for the gate electrode, as in the first conventional embodiment, the heat treatment for reducing the internal stress of the gate electrode should be performed, so that the internal stress of the gate electrode changes as a crystal grows within the tungsten film composing the gate electrode. As a result, a mechanical stress is exerted on the gate insulating film adjacent the gate electrode, which causes a new problem of reduced reliability of the gate insulating film. When the gate insulating film is extremely thin, in particular, the deterioration of the gate insulating film caused by the heat treatment is remarkable.
FIGS. 16(a) and (b) show the results of a TDDB (Time Dependent Dielectric Breakdown) evaluation performed with respect to the reliability of a gate insulating film in a MOS transistor having a gate electrode made of tungsten and a silicon oxynitride film with a thickness of 3.5 nm, of which FIG. 16(a) shows the result of Weibull-plotting the relationship between the charge-to-breakdown value (Qbd) and the cumulative fault probability when a negative bias was applied to the gate electrode and FIG. 16(b) shows the result of Weibull-plotting the relationship between the value Qbd and the cumulative fault probability when a positive bias was applied to the gate electrode. For comparison, there is also shown the case where a gate electrode made of polysilicon is used.
As can be seen from FIG. 16(a), the value Qbd when the negative bias was applied to the gate electrode is higher with the use of the gate electrode made of tungsten than with the use of the gate electrode made of polysilicon. As can be seen from FIG. 16(b), the value Qbd when the positive bias was applied to the gate electrode is lower with the use of the gate electrode made of tungsten than with the use of the gate electrode made of polysilicon.
Thus, in the MOS transistor having the gate electrode made of tungsten and the extremely thin gate insulating film with a thickness of about 3.5 nm, the reliability of the gate insulating film deteriorates when the positive bias is applied to the gate electrode.
To deposit the nitrogen-containing tungsten film composed of the composite of tungsten and tungsten nitride on the gate insulating film by sputtering the target made of tungsten in the nitrogen ambient and diffuse nitrogen contained in the nitrogen-containing tungsten film, as in the second conventional embodiment, a high-temperature, long-term heat treatment should be performed at a temperature of, e.g., 900xc2x0 C. for about 1 minute. In the process of high-temperature heat treatment, nitrogen leaves the gate electrode and the internal stress of the gate electrode changes, while the mechanical stress is exerted on the gate insulating film adjacent the gate electrode. Therefore, the process is not satisfactory in terms of preventing the deterioration of the reliability of the gate insulating film.
In addition, the high-temperature heat treatment for diffusing nitrogen causes another problem of adversely affecting an element formed on the semiconductor substrate, such as a transistor. In particular, a MOS transistor with an extremely small design rule of 0.10 m or less may have its characteristics significantly changed by the high-temperature heat treatment.
In view of the foregoing, it is therefore an object of the present invention to provide a MOS transistor having a gate electrode containing tungsten and an extremely thin gate insulating film with improved reliability without performing a heat treatment.
The present invention has been achieved based on the findings that, if a target made of tungsten is sputtered in an ambient of a gas mixture of an argon gas and a nitrogen gas, a crystal mixture film composed of a mixture of a tungsten crystal and a tungsten nitride crystal can be formed and a heat treatment for reducing the internal stress of the crystal mixture film is no longer necessary and that, if sputtering is performed in an ambient containing a nitrogen gas having a proper partial pressure, a dangling bond formed in a gate insulating film during sputtering is terminated by nitrogen and hence the defect density can be reduced.
Specifically, a method of fabricating a semiconductor device according to the present invention comprises the steps of: forming an insulating film, serving as a gate insulating film, on a semiconductor layer formed on a substrate; and sputtering a target made of tungsten in an ambient of a gas mixture of an argon gas and a nitrogen gas to nitride a surface region of the insulating film and deposit, on the insulating film, a crystal mixture film composed of a mixture of a tungsten crystal and a tungsten nitride crystal and composing at least a part of a gate electrode.
In accordance with the method of fabricating a semiconductor device of the present invention, the target made of tungsten is sputtered in the ambient of the gas mixture of the argon gas and the nitrogen gas. As a result, the nitrogen ions contained in a plasma derived from the nitrogen gas enter the surface region of the insulating film (gate insulating film) so that the surface region is nitrided. Since the surface region of the insulating film, which has been damaged upon collision with tungsten, recovers during the nitriding of the surface region, the reliability of the gate insulating film can be prevented from deteriorating when the positive bias is applied to the gate electrode.
Moreover, since the crystal mixture film composed of the mixture of the tungsten crystal and the tungsten nitride crystal is deposited by sputtering the target made of tungsten in the ambient of the gas mixture of the argon gas and the nitrogen gas, it becomes possible to prevent the changing of the gate electrode from the amorphous state to the crystallized state or the changing of the internal stress of the gate electrode as has been caused by the conventional high-temperature heat treatment. This eliminates the possibility that the mechanical stress is exerted on the gate insulating film and surely improves the reliability of the gate insulating film.
In the method of fabricating a semiconductor device of the present invention, the step of depositing the crystal mixture film is preferably conducted while the substrate is held at 200xc2x0 C. to 500xc2x0 C.
This ensures the crystallization of the tungsten and tungsten nitride composing the crystal mixture film when the crystal mixture film is deposited through sputtering in the ambient of the gas mixture of the argon gas and the nitrogen gas.
In the method of fabricating a semiconductor device of the present invention, the step of depositing the crystal mixture film is preferably conducted in the ambient of the gas mixture of the argon gas and the nitrogen gas such that the weight ratio of nitrogen contained in the crystal mixture film is 10% or less.
Since the tungsten and tungsten nitride composing the crystal mixture film are in the crystallized state, the structure of the crystal mixture film is prevented from changing from the amorphous state to the crystallized state in the step of heat treatment subsequently performed. This prevents the gate insulating film from receiving the mechanical stress resulting from the changing of the structure of the crystal mixture film and thereby improves the reliability of the gate insulating film. This also prevents excess nitrogen from being mixed in the gate insulating film during the deposition of the crystal mixture film as well as an increase in the density of defects produced at the surface of the gate insulating film in contact with the crystal mixture film (gate electrode).
In the method of fabricating a semiconductor device of the present invention, the amount of nitrogen incorporated into the surface region of the insulating film during the step of depositing the crystal mixture film is preferably 1% to 3%.
The arrangement elongates the lifespan of the gate insulating film and reduces the gate leakage current, while preventing an increase in the density of defects at the surface of the gate insulating film in contact with the crystal mixture film (gate electrode).
Preferably, the method of fabricating a semiconductor device of the present invention further comprises, after the step of depositing the crystal mixture film, the step of sputtering the target made of tungsten to deposit, on the crystal mixture film, a tungsten film having a thickness larger than that of the crystal mixture film and composing a part of the gate electrode.
This allows the formation of the gate electrode having a two-layer structure composed of the crystal mixture film and the tungsten film. The resulting gate electrode has reduced resistance compared with the gate electrode composed only of the crystal mixture film and improved reliability compared with the gate electrode composed only of the tungsten film.
In this case, since the tungsten film is deposited by sputtering the target made of tungsten in the ambient of the argon gas, the crystal mixture film and the tungsten film can be deposited continuously by merely changing the atmospheric gas in a chamber. Accordingly, throughput is not reduced compared with the case where the gate electrode composed only of the crystal mixture film is formed.
When the method of fabricating a semiconductor device further comprises the step of depositing the tungsten film, it preferably comprises, after the step of depositing the tungsten film, the step of performing a CVD or sputtering process at a temperature of 500xc2x0 C. or less to deposit, on the tungsten film, an upper insulating film serving as a mask pattern for forming the gate electrode.
In forming the gate electrode having the two-layer structure composed of the crystal mixture film and the tungsten film, the mask pattern for etching the multilayer structure composed of the crystal mixture film and the tungsten film is required. In this case, if the upper insulating film forming the mask pattern is deposited at a temperature of 500xc2x0 C. or lower, the heat during the deposition of the upper insulating film prevents the production of electric defects in the gate insulating film.
Preferably, the method of fabricating a semiconductor device of the present invention further comprises the steps of: sputtering the target made of tungsten in an ambient of the argon gas to deposit, on the crystal mixture film, a first tungsten film composing a part of the gate electrode; sputtering the target made of tungsten in the ambient of the gas mixture of the argon gas and the nitrogen gas to deposit, on the first tungsten film, an upper crystal mixture film composed of the mixture of the tungsten crystal and the tungsten nitride crystal and composing a part of the gate electrode; and sputtering the target made of tungsten in the ambient of the argon gas to deposit, on the upper crystal mixture film, a second tungsten film composing a part of the gate electrode.
The arrangement allows the formation of the gate electrode having a four-layer structure composed of the crystal mixture film, the first tungsten film, the upper crystal mixture film, and the second tungsten film. As a result, the crystal grows individually on each of the first and second tungsten films even if a heat treatment is performed thereafter. This suppresses an increase in the diameter of a crystal grain under growth and surely prevents the deterioration of the reliability of the gate insulating film.
A semiconductor device according to the present invention comprises: a gate insulating film formed on a semiconductor layer on a substrate; and a gate electrode having a crystal mixture film deposited on the gate insulating film and composed of a mixture of a tungsten crystal and a tungsten nitride crystal.
In the semiconductor device according to the present invention, the gate electrode has the crystal mixture film composed of the mixture of the tungsten crystal and the tungsten nitride crystal, so that it is no longer necessary to perform such a high-temperature treatment as has been performed conventionally. As a result, the gate electrode is prevented from changing from the amorphous state to the crystallized state or the internal stress of the gate electrode is prevented from changing during the high-temperature heat treatment. This eliminates the possibility that the mechanical stress is exerted on the gate insulating film and surely improves the reliability of the gate insulating film.
In the semiconductor device of the present invention, the weight ratio of nitrogen contained in the crystal mixture film is preferably 10% or less.
In the arrangement, the structure of the crystal mixture film is prevented from changing from the amorphous state to the crystallized state in the heat treatment subsequently performed. This eliminates the possibility that the mechanical stress is exerted on the gate insulating film due to the changing of the structure of the crystal mixture film and surely improves the reliability of the gate insulating film.
In the semiconductor device of the present invention, the gate electrode preferably has a multilayer structure composed of the crystal mixture film and a tungsten film deposited on the crystal mixture film and having a thickness larger than that of the crystal mixture film.
This provides a gate electrode having reduced resistance compared with the gate electrode composed only of the crystal mixture film and having improved reliability compared with the gate electrode composed only of the tungsten film.